Preheat oven to 350°F (177°C). Generously spray a 12-count muffin pan with nonstick spray. If using muffin liners, spray them with nonstick spray. The oatmeal cups stick to the liners a bit regardless, so I recommend skipping them.

In a large bowl, whisk all of the ingredients together, except for the optional add-ins and coarse sugar. Using a spoon or silicone spatula, fold in the chocolate chips and nuts (if using). Batter will be a little liquidy. Spoon batter evenly into muffin cups, making sure both the oats/add-ins and liquid are in each. Fill all the way to the top. See photo above. If desired for extra crunch, sprinkle the tops with coarse sugar.

Bake for 28–30 minutes or until the edges are lightly browned and tops appear set. If the tops are browning very quickly, tent a piece of aluminum foil over the muffin pan.

Cool for 5-10 minutes before serving.

Cover leftover oatmeal cups tightly and refrigerate for up to 1 week.